Re: [PATCH 2/2] riscv: Fix text patching when IPI are used

[Samuel Holland <samuel.holland@sifive.com>]

Hi Alex,

On 2024-02-28 11:51 AM, Alexandre Ghiti wrote:
> For now, we use stop_machine() to patch the text and when we use IPIs for
> remote icache flushes (which is emitted in patch_text_nosync()), the system
> hangs.
> 
> So instead, make sure every cpu executes the stop_machine() patching
> function and emit a local icache flush there.
> 
> Co-developed-by: Björn Töpel <bjorn@rivosinc.com>
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  arch/riscv/include/asm/patch.h |  1 +
>  arch/riscv/kernel/ftrace.c     | 42 ++++++++++++++++++++++++++++++----
>  arch/riscv/kernel/patch.c      | 18 +++++++++------
>  3 files changed, 50 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/patch.h b/arch/riscv/include/asm/patch.h
> index e88b52d39eac..9f5d6e14c405 100644
> --- a/arch/riscv/include/asm/patch.h
> +++ b/arch/riscv/include/asm/patch.h
> @@ -6,6 +6,7 @@
>  #ifndef _ASM_RISCV_PATCH_H
>  #define _ASM_RISCV_PATCH_H
>  
> +int patch_insn_write(void *addr, const void *insn, size_t len);
>  int patch_text_nosync(void *addr, const void *insns, size_t len);
>  int patch_text_set_nosync(void *addr, u8 c, size_t len);
>  int patch_text(void *addr, u32 *insns, int ninsns);
> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> index f5aa24d9e1c1..5654966c4e7d 100644
> --- a/arch/riscv/kernel/ftrace.c
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -8,6 +8,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/uaccess.h>
>  #include <linux/memory.h>
> +#include <linux/stop_machine.h>
>  #include <asm/cacheflush.h>
>  #include <asm/patch.h>
>  
> @@ -75,8 +76,7 @@ static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
>  		make_call_t0(hook_pos, target, call);
>  
>  	/* Replace the auipc-jalr pair at once. Return -EPERM on write error. */
> -	if (patch_text_nosync
> -	    ((void *)hook_pos, enable ? call : nops, MCOUNT_INSN_SIZE))
> +	if (patch_insn_write((void *)hook_pos, enable ? call : nops, MCOUNT_INSN_SIZE))
>  		return -EPERM;
>  
>  	return 0;
> @@ -88,7 +88,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
>  
>  	make_call_t0(rec->ip, addr, call);
>  
> -	if (patch_text_nosync((void *)rec->ip, call, MCOUNT_INSN_SIZE))
> +	if (patch_insn_write((void *)rec->ip, call, MCOUNT_INSN_SIZE))
>  		return -EPERM;
>  
>  	return 0;
> @@ -99,7 +99,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  {
>  	unsigned int nops[2] = {NOP4, NOP4};
>  
> -	if (patch_text_nosync((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
> +	if (patch_insn_write((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
>  		return -EPERM;
>  
>  	return 0;
> @@ -134,6 +134,40 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
>  
>  	return ret;
>  }
> +
> +struct ftrace_modify_param {
> +	int command;
> +	atomic_t cpu_count;
> +};
> +
> +static int __ftrace_modify_code(void *data)
> +{
> +	struct ftrace_modify_param *param = data;
> +
> +	if (atomic_inc_return(&param->cpu_count) == num_online_cpus()) {
> +		ftrace_modify_all_code(param->command);
> +		/*
> +		 * Make sure the patching store is effective *before* we
> +		 * increment the counter which releases all waiting cpus
> +		 * by using the release version of atomic increment.
> +		 */
> +		atomic_inc_return_release(&param->cpu_count);
> +	} else {
> +		while (atomic_read(&param->cpu_count) <= num_online_cpus())
> +			cpu_relax();
> +	}
> +
> +	local_flush_icache_all();
> +
> +	return 0;
> +}
> +
> +void arch_ftrace_update_code(int command)
> +{
> +	struct ftrace_modify_param param = { command, ATOMIC_INIT(0) };
> +
> +	stop_machine(__ftrace_modify_code, &param, cpu_online_mask);
> +}
>  #endif
>  
>  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> index 0b5c16dfe3f4..82d8508c765b 100644
> --- a/arch/riscv/kernel/patch.c
> +++ b/arch/riscv/kernel/patch.c
> @@ -188,7 +188,7 @@ int patch_text_set_nosync(void *addr, u8 c, size_t len)
>  }
>  NOKPROBE_SYMBOL(patch_text_set_nosync);
>  
> -static int patch_insn_write(void *addr, const void *insn, size_t len)
> +int patch_insn_write(void *addr, const void *insn, size_t len)
>  {
>  	size_t patched = 0;
>  	size_t size;
> @@ -211,11 +211,9 @@ NOKPROBE_SYMBOL(patch_insn_write);
>  
>  int patch_text_nosync(void *addr, const void *insns, size_t len)
>  {
> -	u32 *tp = addr;
>  	int ret;
>  
> -	ret = patch_insn_write(tp, insns, len);
> -
> +	ret = patch_insn_write(addr, insns, len);
>  	if (!ret)
>  		flush_icache_range((uintptr_t) tp, (uintptr_t) tp + len);

This only happens to compile because flush_icache_range() is a macro that
ignores its parameters. You could replace tp with addr in this line as well, but
that seems like more of a cosmetic change and should be a separate patch (like
in [1] which covers both related functions) if you respin this.

Regards,
Samuel

[1]:
https://lore.kernel.org/linux-riscv/20240212025529.1971876-8-samuel.holland@sifive.com/

>  
> @@ -232,15 +230,21 @@ static int patch_text_cb(void *data)
>  	if (atomic_inc_return(&patch->cpu_count) == num_online_cpus()) {
>  		for (i = 0; ret == 0 && i < patch->ninsns; i++) {
>  			len = GET_INSN_LENGTH(patch->insns[i]);
> -			ret = patch_text_nosync(patch->addr + i * len,
> -						&patch->insns[i], len);
> +			ret = patch_insn_write(patch->addr + i * len, &patch->insns[i], len);
>  		}
> -		atomic_inc(&patch->cpu_count);
> +		/*
> +		 * Make sure the patching store is effective *before* we
> +		 * increment the counter which releases all waiting cpus
> +		 * by using the release version of atomic increment.
> +		 */
> +		atomic_inc_return_release(&patch->cpu_count);
>  	} else {
>  		while (atomic_read(&patch->cpu_count) <= num_online_cpus())
>  			cpu_relax();
>  	}
>  
> +	local_flush_icache_all();
> +
>  	return ret;
>  }
>  NOKPROBE_SYMBOL(patch_text_cb);